Protection of the environment and human beings has become more and more important. Buildings, cars, buses, aircraft, household appliances and industrial machinery have noise producing components such as engines, motors, gears, transmissions etc. In order to protect individuals from such noise, the noise generating components and the transmission path of the noise to a human being have been investigated with the purpose of reducing the generated noise at the source and of reducing the noise transmitted from the source to human beings.
Testing of acoustic properties of noise generating and noise transmitting media such as mechanical structures and air or other fluids is an important part of the process of noise reduction. In complex structures with several noise sources, such as mentioned above, it can be complicated to identify noise sources and transmission paths and their contributions to the perceived noise.
Computerized methods exist for analyzing physical structures, and mathematical models of analyzed structures can be made. Acoustical tools exist for simulating acoustic properties of portions of a human being, such as Mouth Simulator type 4227, Ear Simulators types 4185 and 4195, Head and Torso Simulator types 4100 and 4128, all from Brüel & Kjaer Sound and Vibration Measurement A/S. All of these are intended for use in analyzing sound at different stages in its “normal” forward transmission from the source to a human being.
The transfer function for sound from a sound source to a point of measurement is often expressed as the acoustical transfer function or transfer impedance Zt defined as Zt=p/Q, where Q is the volume velocity from the sound source, and p is the sound pressure at the point of measurement resulting from the volume velocity generated by the sound source. In most cases the analyzed mechanical and acoustical transmission media are reciprocal, which means that the acoustical transfer function is the same both for forward and reverse transmission. In other words, if the sound source and the measuring microphone are interchanged, whereby the transmission of sound through the structure is reversed, and boundary conditions remain unchanged, then the acoustical transfer impedance is unaffected, ie the “forward” acoustical transfer impedance and the “reverse” acoustical transfer impedance are identical.
For measurements of the acoustic transfer impedance it is necessary to know the volume velocity of the output sound signal. This is true both for measurements in the forward direction and in the reverse direction. It is known to use this fact when analyzing the transmission of sound, whereby a sound source is placed in a position that is normally occupied by a human being, ie a “listening” position, and a microphone is placed in the normal position of the sound source. This has distinct advantages when identifying sound sources and tracking the noise on its path from the source to listening position.
When measuring the forward transmission path a Head and Torso Simulator type 4100 from Brüel & Kjaer Sound and Vibration Measurement A/S can be placed in the listening position, whereby very realistic measurements of the forward transmission path can be obtained. However, when measuring the reverse transmission path with today's technology one still has to use a traditional sound source in the listening position, and traditional loudspeakers suffer form the drawback that they do not simulate any acoustic properties of a human being. The Mouth Simulator type 4227 and the Torso Simulator type 4128, both from Brüel & Kjaer Sound and Vibration Measurement A/S, each simulates the acoustic properties of the mouth of a human being very well, but this property of the commercially available simulators is irrelevant to measurements using the reverse transmission path. There is thus a need for a sound source for use in such measurements.
DE 2 716 345 discloses a dummy head with two built-in loudspeakers for emitting stereophonic sound through the two ears of the dummy head; in particular stereophonic sound recordings made with a dummy head having microphones in its ears.
U.S. Pat. No. 4,631,962 discloses an artificial head measuring system composed of geometric bodies for simulating acoustic properties of a human head. Microphones are disposed in the auditory canals of the artificial head. In relation to the instant invention the artificial head measuring system of U.S. Pat. No. 4,631,962 corresponds to the above-mentioned Head and Torso Simulator type 4100 from Brüel & Kjaer Sound and Vibration Measurement A/S.
JP 07 264632 discloses a dummy head with a pair of microphones for making stereophonic sound recordings and a pair of cameras for making stereoscopic video recordings simultaneously with the sound recordings.
JP 60 254997 discloses a system including a dummy mannequin with microphones in its ears for measuring acoustic transfer characteristics e.g. in an automobile using the forward transmission path.